Comparison of the Reactivity of PDMS with Terminal Allyl and Vinyl Groups in the Hydrosilylation Reaction

Controlling the hydrosilylation rate during the curing of polydimethylsiloxanes (PDMS) is essential for regulating processing parameters and the properties of silicone materials. This study explores the possibility of achieving such control through differences in the reactivity of terminal allylsilyl and vinylsilyl groups. Difunctional PDMS bearing terminal allyl, vinyl, or hydride groups, together with a narrowly dispersed monofunctional PDMS bearing a terminal hydride group, were synthesized for model experiments. The starting compounds and hydrosilylation products were characterized by 1H and 29Si NMR spectroscopy and gel permeation chromatography. When allylsilyl and vinylsilyl groups were present in the same reaction mixture, hydrosilylation preferentially involved vinylsilyl groups. Additional model experiments confirmed the higher relative reactivity of vinylsilyl groups and revealed competing isomerization of the allylic double bond, which likely contributes to the slower hydrosilylation of allylsilyl groups. These findings indicate that differences in functional group reactivity can be used to control the sequence of hydrosilylation reactions and guide the molecular design of silicone materials with tailored properties.